How to build an 8 MHz oscillator with a crystal?

Question

I want to build an oscillator and have an 8 MHz crystal laying around, so I looked around to find a circuit to make an oscillator with it. I found this schematic:

However, I do not have a 4069 laying around, so I replaced it with a 7400 with one input pin bound to +5 V. My (naive?) brain checks the switching speed of the 7400 and max. 22 ns should be well in range of 8 MHz.

Still, the circuit does nothing. So I have two questions:

1. How important is it that I use a 4069UB ?
2. Is there another way of making an 8 MHz oscillator with the crystal and some 74xx logic gates?

Answer 1

How important is it that I use a 4069UB

The important bit is the "UB" after "4069". It stands for unbuffered and, an unbuffered device is needed. A 7400 is "buffered" i.e. it won't work as intended.

Is there another way of making an 8MHz oscillator with the crystal and some 74xx logic gates?

Providing it inverts and is unbuffered then it should work. Use CMOS parts though because TTL devices will have input impedances that are too low to get reliable oscillation through a crystal.

You might also add this resistor here: -

Answer 2

1. It does not have to be 4069UB, as any unbuffered CMOS inverter will work with that circuit.

For example, a 74HCU04 should work without modifications.

2. Yes. But as there are many types of gates and logic types for gates, a listing them all here would be quite a task.

Technically, NAND gates and inverters tend to be used. Unbufferd devices are preferable. Each different type of logic needs different external component values. 74LS should be possible but it may require completely different oscillator topology with two LS inverters and very low value resistors in the kilo-ohm region, 74HC are buffered may have too much gain so external series resistor of few hundred ohms may be required, 74HCU and 4000 series should be roughly equal.

Answer 3

How important is it that I use a 4069UB ?

The 7400 series have three disadvantages:

1. Significant input current. The 74LS, 74ALS et cetera have lower requirements
2. Asymmetrical input thresholds
3. Asymmetrical output drive current

THe CMOS 4000, 74HCxx, and 74LVCxx series have

1. Low input drive current
2. Symmetrical input thresholds
3. Symmetrical output drive current if buffered. Only unbuffered inverters have symmetrical drive current

The crystal is a sinusoidal device and so a linear amplifier performs better than a switching one. The unbuffered CMOS inverters are a single pair of FETS. This provides the lowest propagation delay and a lower gain. It is an inexpensive approximation to a linear amplifier. To square up the sinusoid into a clock a following inverter is used.

Is there another way of making an 8 MHz oscillator with the crystal and some 74xx logic gates?

Any amplifier can be made to oscillate. All logic gates/inverters/buffers are amplifiers and so can be made to oscillate. The unbuffered CMOS temds to be preferred.